One of the most active immunoadjuvants is Freund's Complete Adjuvant which is a water-oil emulsion consisting of 10% Arlacel A and 90% mineral oil containing whole killed mycobacterial cells. A vaccine is formulated with Freund's Complete Adjuvant by incorporating the antigen in the aqueous phase. Therapeutic applications of Freund's Complete Adjuvant, however, have been prevented due to accompanying toxic side effects such as local granulomas, endotoxic shock, and adjuvant-induced polyarthritis. Subsequently, the minimal active structure of mycobacteria has been determined by Ellouz et al., Biochem. Biophys. Res. Commun., 59, 1317 (1974) and by Kotani et al., Biken J., 18, 105 (1975) to be a peptidoglycan fragment of the cell wall, more specifically, a muramyl dipeptide, namely, N-acetylmuramyl-L-alanyl-D-isoglutamine (MDP). The addition of synthetic MDP to an emulsion of Freund's incomplete adjuvant (90% mineral oil and 10% Arlacel A) containing an antigen increases the level of antibodies against the antigen (humoral response) and induces delayed hypersensitivity (cellular immunity).
The effects of various structural modifications of the dipeptidyl moiety of MDP on biological activity have been reported, although studies on the effects of modifications of the saccharide moiety have been limited.